1. Field of the Invention
Many types of industrial machinery have reciprocative or oscillatory mechanisms actuated pneumatically or by gravity or srings. A majority of such devices are operated by reciprocative pneumatic actuators, and in the main, the present disclosure describes problems involved in the use of such actuators and the solution of those problems. It is intended however that the use of the new improved hydraulic cartridges described herein not be limited to pneumatic actuators.
2. Description of the Prior Art Regarding Actuators
The usual reciprocative pneumatic actuator consists of a single cylinder with reciprocative piston and rod, the cylinder being closed at its ends by heads through which compressed air is supplied and exhausted. When compressed air enters the cylinder it tends to move the air piston so quickly and with such force that the piston and any moving load attached to it strike with heavy impact at the ends of the stroke. The impact resulting from an air piston striking the heads can be severe enough to damage the actuator and the mechanism driven by it. The impact can easily be great enough to shear off the mounting bolts of the actuator completely, thereby releasing the same from its base. The consequent danger to personnel and equipment is readily understandable, yet many pneumatic actuators are operated just as they are received from the seller, with no safeguarding against impact, so that direct impact of the piston against one or both of the heads often provides the only means of stopping the piston and its attached load. Such actuators must be operated at low speed at much less than their potential power output to save them from destruction. Operation at a safe low speed is usually accomplished by restricting the flow of air to or from the actuator, most pneumatic actuators being operated at speeds much lower than would be most economical for the moving load attached to them, because the operator adjusts the airflow low enough to be on the safe side. An expensive loss in efficiency occurs then because the full stroke of the air piston must be traveled at low speed, with consequent low productive capacity of the device.
3. Prior Art Regarding Pneumatic Cushions
Pneumatic actuators may be purchased equipped with one or two pneumatic cushions, each of which consists of a valving device for closing the exhaust passage at one of the cylinder to trap air to decelerate the piston as it reaches the end of its stroke. This type of cushion is effective only for slow moving or lightly loaded pistons. The trapped air acts like a spring and has relatively little decelerative action because it is not compressed to an effective braking pressure until a small fraction of an inch before the piston reaches the end of its stroke.
4. Prior Art Regarding External Hydraulic Decelerators
Moving mechanisms actuated by pneumatic actuators are sometimes equipped with external hydraulic decelerators to decelerate the mechanism at the ends of its stroke to permit a high operating speed. One type of such decelerator unit is described in the enclosed bulletin entitled "Cushioneer". It consists of a hydraulic cylinder with a reciprocative fluid damped plunger and is usually mounted at a distance from the actuator so that some portion of the moving mechanism strikes the plunger.
A notable point is that with such an arrangement, each time the moving load is brought to a stop, the hydraulic decelerator converts to heat a large percentage of the air power expended during the stroke, the heat being generated by friction within the hydraulic fluid as it is forced through restrictive flow passageways. If operation of the pneumatic actuator is fast and continual, the decelerator becomes so hot that the elastomeric seals it contains may be damaged unless the decelerator is cooled by some external means that dissipates the heat. A measure of cooling can be effected by leading the exhaust air from the actuator to impinge against the outside surface of the body of a decelerator as illustrated at upper right on page 2 of the above referenced Cushioneer bulletin, but it can be seen that at best this type of cooling is not only a makeshift arrangement but all of the energy taken from the moving load is lost.
A second notable point is that oftentimes the need for cooling is neither understood nor apparent to the installer of a decelerator, and unfortunatley, the instructions furnished by decelerator manufactureres are often disregarded. When external decelerators are insufficiently cooled and are operated at high speed and/or high loads, the hydraulic fluid becomes overheated and reduced in viscosity so that the moving load may not be decelerated sufficiently to prevent the moving parts from striking with heavy impact. Frequent shut-downs are necessary to repair damage resulting from this type of operation.
A third point is that situations often occur in the field of hydraulic decelerators where considerable technical "know how" is required for a user or a salesman to choose and install a decelerator properly. The responsible person must be able to calculate load weight and velocity, and to substitute such values along with actuator requirement in a mathematical formula to work out the result correctly. The majority of persons involved with actuator installation cannot do this.
A fourth point is that if the installer is not skilled and careful, he may mount a hydraulic decelerator insecurely or in a misaligned position so that the impinging load applies a lateral load on the plunger. If this doesn't actually bend the plunger, it soon causes excessive wear on the plunger slide bearing and the hydraulic piston.
Because of the problems mentioned above, many years of development and testing work have been spent by applicant in an effort to construct a compact hydraulic decelerator with enough energy absorption capacity to be installed in both ends of pneumatic actuators to permit high operating speed. The restriction in space between the actuator piston rod and the inside of the actuator cylinder has been a major problem, as have leakage of the hydraulic fluid, tendency of the compressed air to enter the fluid and form bubbles, and overheating of the hydraulic fluid with consequent short life of the fluid seals.